Check modified request headers at the mirrored backend.

[Treenhan]

What would you like to be added:
Checks if the headers are mirrored in the request sent.

Why this is needed:
While developing conformance tests, we discovered an issue with the ExpectMirroredRequest method (https://github.com/kubernetes-sigs/gateway-api/blob/main/conformance/utils/http/mirror.go#L32). This method verifies that a request is mirrored properly. However, it was only checking that the path matched for mirrored requests. It did not have checks in place for the headers.

This is problematic because headers can also be modified or added to requests. To have fully conformant request mirroring, the test method needs to account for headers as well as the path.

More context: #2101 (comment)

/kind feature
/area conformance

[Treenhan]

Hi @shaneutt can I work on this one?

[shaneutt]

Seems like a helpful improvement:

/triage accepted

Please do feel free to work on this, let us know if you need help or run into any trouble.

/assign @Treenhan

[shaneutt]

@Treenhan Heya! 👋

Just checking in, everything going OK have any questions about this one?

[Treenhan]

Hi @shaneutt thank you for asking, do you know what happens to the response from the mirrored backend? Do you know if the gateway forwards it back to the original client? If so, how can the client see this response?